514 ELECTRICAL MEASUREMENTS 



In the small sizes (up to and including 25 amp.) the cus- 

 tomer's entire current is taken in through the leads, LL, and to 

 the heater strips via the spring hinges, h, and flexible connecting 

 strips, k. 



The indicator tube is of glass, annealed so that it will bear 

 handling and not be subject to changes due to stresses in the glass; 

 the two bulbs, 61 and 6 2 , which are nearly equal in volume, con- 

 tain air. The U-tube connecting them contains concentrated 

 sulphuric acid in such an amount and so adjusted in the tube 

 that when the indicator is cold and set ready to begin to operate, 

 the level of the liquid is at d, so that it is just on the point of flow- 

 ing into the index tube, iz. Sulphuric acid is used because it 

 "wets" the glass, is very heavy, flows readily, is hygroscopic, 

 and expands comparatively little with rise of temperature. To 

 prevent accidental transfer of air from 61 to 6 2 , or vice versa, 

 especially when the indicator is set, the tube is constricted to a 

 capillary at g and g' and two traps are provided at t and t f . 



The heater strips are of an alloy of high resistivity, which is 

 but little affected by temperature; the strips are of very thin 

 metal and are made to embrace closely the cylindrical glass 

 bulb, 61, by means of screw clamps. In the small-sized indica- 

 tors, where it is necessary to carry the heater strips around the 

 bulb a number of times, a non-inductive form is used. The 

 object of this construction is to prevent the turns drawing to- 

 gether when a short-circuit occurs; if this should happen the strips 

 would very likely be burned out or their intimacy of contact 

 with the glass so altered that an error would be introduced. The 

 corrugated copper terminals form somewhat flexible electrical 

 connections to the heater proper. In indicators having a range 

 of 35 amp. and above, shunts are used, the heater strips being of 

 the 15-amp. type. In the shunted instrument, to insure per- 

 manency of calibration, it is essential that all the electrical 

 joints be soldered. 



The indicator is set by raising the lower end of the tube board, 

 M, on which the above described members are mounted, until 

 it is somewhat above the spring hinges, h, on which it is pivoted. 

 This allows the liquid in the index tube iz to drain back into the 

 U-tube; when thoroughly drained and the board is lowered to its 

 normal position, the [/-tube is filled with liquid up to d. 



